Evaporation from simulated soil pores: Effects of wettability, liquid islands, and breakup

Abstract

Two-thirds of worldwide water withdrawals are for agriculture; this represents a key challenge in the food–energy– water nexus. In the U.S. Central High Plains, the Ogallala aquifer—the primary water source for agriculture—is depleting. Reducing water evaporation from soil provides an opportunity to decrease irrigation, thus conserving water resources. In this study, evaporation phenomena of 4 µL sessile water droplets were analyzed from a simulated soil pore created with 2.38 mm hydrophilic glass and hydrophobic Teflon beads. The experiments were conducted at 22◦C and 60% relative humidity. Two geometries were studied: symmetric (i.e., center-to-center spacing between the beads of 3.1 mm) and asymmetric (i.e., center-to-center spacings of 2.7 mm and 2.8 mm). Evaporation phenomena were recorded using a high-speed camera and evaporation times were recorded. Evaporation was faster from the hydrophilic pore (e.g., 34 min) compared to the hydrophobic pore (e.g., 42 min) in the symmetric configuration, due in part to greater air–water contact areas. Spacing between the beads affected evaporation, as evaporation rates to completely evaporate the droplet were slower for hydrophilic (e.g., 44 min) and hydrophobic (e.g., 56 min) pores in the asymmetric configuration. The formation of liquid island, projected area, liquid island contact angles, volume, and rupture strength of droplet were analyzed for all four combinations. The droplet retained its initial projected area, wetted length, and volume for a certain time during evaporation from Teflon pores (e.g., 5–10 min), while these parameters decreased simultaneously during evaporation from glass pores.